To say that Bilbo's breath was taken away is no description at all. There are no words left to express his staggerment, since Men changed the language that they learned of elves in the days when all the world was wonderful. Bilbo had heard tell and sing of dragon-hoards before, but the splendour, the lust, the glory of such treasure had never yet come home to him. His heart was filled and pierced with enchantment and with the desire of dwarves; and he gazed motionless, almost forgetting the frightful guardian, at the gold beyond price and count.

From Not At Home - the Arkenstone:

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...At last he looked down upon it and he caught his breath. The great jewel shone before he feet of its own inner light, and yet, cut and fashioned by the dwarves, who had dug it from the heart of the mountain long ago, it took all light that fell upon it and-changes it into ten thousand sparks of white radiance shot with glints of the rainbow. Suddenly Bilbo's arm went towards it drawn by it enchantment. His small hand would not close about it for it was a large and heavy gem; but he lifted it, shut his eyes, and put it in his deepest pocket. "Now I am a burglar indeed!" thought he. "But I suppose I must tell the dwarves about it-some time. The did say I could pick and choose my own share; and I think I would choose this, if they took all the rest!" All the same he had an uncomfortable feeling that the picking and choosing had not really been meant to include this marvellous gem, and that trouble would yet come of it.

Of course he did not tell them anything. From The Gathering of the Clouds - Thorin the Greedy:

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...But also he did not reckon with the power that gold has upon which a dragon has long brooded, nor with dwarvish hearts. Long hours in the past days Thorin had spent in the treasury, and the lust of it was heavy on him. Though he had hunted chiefly for the Arkenstone, yet he had an eye for many another wonderful thing that was lying there, about which were wound old memories of the labours and the sorrows of his race.

As I mentioned in my response to ShireHorse, it is clear from several references that Tolkien did intend Thorin to become (at this point - and not before!) fallen because of his pride and his greed. But this is not quite what he had written - this happens after sevral days and not immediately; and is qualified by the curse of gold on which a dragon has brooded (Shippey discusses this theme as well); and by the memories of the labours and sorrows of his race. And even so, Thorin's actions are still fair and measured. The only time Tolkien says that his judgment was actually clouded because of the 'bewilderment of treasure' is when after being betrayed and blackmailed into declaring he will ransome the Arkenstone for Bilbo's share, he still considers whether together with Dain's army he could recapture it without bargaining for it.As a last thing, regarding The Hoard - as geordie noted, this poem originated in the 20s, at the time when the Dwarves were viewed by Tolkien as prounncedly evil (if politically often neutral). Even after The Hobbit was published, Tolkien's view of them was softened, but still quite ambivalent, as I've discussed here (questions 8-13, and 16-17). The heroics of Azaghâl were inspired by Thorin, and not the other way round, if you understand what I mean.